Digestive System Handout - Part 1 PDF

TCM 115 – Basic Anatomy and Physiology DIGESTIVE (ALIMENTARY) SYSTEM – HANDOUT (part 1) UPPER DIGESTIVE TRACT Digestive tract (alimentary) is also known as the gastrointestinal system which is composed of the following: a) oral cavity (mouth) b) pharynx c) esophagus...

TCM 115 – Basic Anatomy and Physiology DIGESTIVE (ALIMENTARY) SYSTEM – HANDOUT (part 1) UPPER DIGESTIVE TRACT Digestive tract (alimentary) is also known as the gastrointestinal system which is composed of the following: a) oral cavity (mouth) b) pharynx c) esophagus d) stomach e) small intestine f) large intestine Although the entire system is composed of hollow, tube-like structure, there are several associated structures that are inseparable from GI tract. Those structures are: a) teeth b) tongue c) salivary glands d) liver e) gallbladder f) pancreas Overall digestive system performs the following six functions: 1. Ingestion – intake of food 2. Secretion – approximately 7 liters of fluid is released into digestive system 3. Mixing and propulsion 4. Digestion – mechanical and chemical 5. Absorption of nutrients 6. Elimination of waste – defecation Layers of digestive system There are four layers of the digestive tube: mucosa (inner most), submucosa, muscularis and serosa (outermost) Mucosa – innermost layer Muscosa is composed of 3 sublayers, epithelium, lamina propria and muscularis mucosae. Epithelium – nonkeratinized stratified squamous epithelium in mouth, pharynx and anus; the rest of the GI tract is lined with simple columnar epithelium which is good for secretion and absorption. Also there are many exocrine glands – secretion of mucus Lamina propria – areolar connective tissue with abundant lymphatics and blood vessels. It contains lymphatic nodules (mucosa associated lymphatic tissue – MALT) in tonsils, small intestine –Peyer’s patches and appendix Muscularis mucosae – creates numerous folds of mucosa so that food comes in contact with absorptive epithelium. Submucosa Areolar connective tissue, many blood vessels and lymphatics. It also contains submucosal plexus of Meissner (nerves that are part of the enteric nervous system). These nerves regulate movement of mucosa and vasoconstriction of blood vessels as well as secretion of glands. Muscularis Voluntary skeletal muscles are present in mouth, pharynx and proximal esophagus. It is gradually replaced by smooth muscle tissue that becomes arranged in 2 layers, inner (circular) and outer (longitudinal) layer. Exceptionally, the stomach will have the third, obliquely running layer. Between inner and outer muscle layers we have the plexus of Auerbach, nervous fibers that control motility of food through the GI tract. Serosa Outermost layer that covers the GI tract. It is alveolar connective tissue and simple squamous epithelium. Adventitia covers esophagus, but inferior to the diaphragm, it is called the peritoneum. Mouth (oral or buccal cavity) Oral cavity is marked by lips, cheeks, hard and soft palate, muscles of the floor of oral cavity and tongue. Lips – Labium superior and labium inferior. Connected to gums by small midline folds of oral mucosa – labial frenulum Cheeks – composed of buccinator muscle and subcutaneous fat. Behind teeth and gums is the oral cavity proper. It communicates with the oropharynx through the opening which is called fauces. Hard and soft palate form the roof of the oral cavity. Salivary glands There are numerous small glands throughout the oral cavity, but only 3 major (paired) glands are observed. Their names are:  parotid glands,  submandibular  sublingual All of them have their drainage ducts that open directly into oral cavity. Saliva contains lysosyme and salivary amylase. Composition of saliva Saliva is 99.5% water and only 0.5% solute. It contains bacteriolytic enzyme lysozyme as well as small amount of salivary amylase that breaks down starch molecules Salivation It is controlled by autonomic nervous system and it accounts for 1 – 1.5 l per day. Parasympathetic fibers stimulate increased production of saliva, but sympathetic innervation reduces it Different chemicals contained in food also stimulate salivation; salivation can be increased if other stimuli are affecting our body (such as sight, smell or even thought) Saliva assists in the formation of bolus, and it rinses oral cavity after the meal Digestion in oral cavity  Mechanical (chewing)  Chemical (lingual lipase and salivary amylase)  Formation of bolus Tongue (lingua) Three main parts are the root, body and apex. Dorsal surface of the tongue is covered with papillae which might have on theirs surfaces taste buds. and IX carry taste signals to the brain. Bulk of the tongue is composed of striated muscles that attach the tongue to the floor of the oral cavity. Glands on the dorsum of the tongue secrete lingual lipase Teeth Humans have 2 sets of teeth during their lives: deciduous (primary, milk or baby teeth) and permanent teeth. There are only 20 primary teeth that are replaced by 32 permanent teeth. Based on their shape, size and position permanent teeth are recognized as: Incisors – 2, Canine – 1, Premolars – 2 and Molars –3. Primary teeth are lost between ages 6 – 12 and are gradually replaced by permanent teeth. Permanent teeth are inserted into their own sockets within upper or lower jaws (alveoli). Each tooth is composed of: crown, neck and root. Bulk of the tooth is dentin which gives tooth its basic shape and rigidity. The crown is covered with enamel. A tooth also has the central (pulp) cavity which contains nerve fibers and blood vessels. Pharynx It is a muscular tube that extends from the base of the skull to approximately C6 vertebra. It is divided into three sections: epipharynx (nasopharynx), oropharynx (midpharynx) and hypopharynx (laryngopharynx) as discussed previously. Through the fauces food passes from oral cavity into pharynx, inferiorly pharynx is continuous with esophagus. One of the more important functions of pharynx is to commence the peristaltic wave and to be involved in deglutition (swallowing). Deglutition – swallowing Movement of bolus from oral cavity through pharynx into esophagus is a complex act that is termed deglutition. It occurs in three steps: 1) Voluntary stage (oral cavity to oropharynx) 2) Pharyngeal stage (bolus passes from pharynx into prox. esophagus) 3) Esophageal stage (esophagus into stomach) Voluntary stage is mostly accommodated by movement of the tongue up and backwards. At this stage swallowing could be stopped. Once the bolus entered the oropharynx, this act becomes involuntary. It involves the movement of the following structures: 1. Larynx (elevated superiorly and anteriorly) 2. Epiglottis closes entrance into larynx 3. Vocal cords are adducted (day thanh quan khep lai) 4. Respiration is temporarily interrupted (ho hap bi gian doan tam thoi) 5. Upward movement of larynx stretches proximal esophagus, so that the food entrance into esophagus is facilitated. (de dang hơn) Esophageal stage is also involuntary. It requires peristaltic wave which propels bolus towards the stomach. It is accomplished by synchronized contraction/relaxation of circular and longitudinal muscles of its muscular layer. Reversed movement of bolus is prevented by the superior and inferior esophageal sphincter Esophagus It is approximately 25 cm long muscular tube that extends from hypopharynx to stomach. It passes through the thorax (mediastinum), pierces diaphragm (level of T10 vertebra) and terminates within the abdominal cavity. Upper esophageal sphincter (cricopharyngeus muscle) regulates intake of bolus from pharynx into esophagus, while the lower esophageal sphincter regulates emptying of the esophagus and passage of bolus into the stomach. We used the word bolus for food that is passed from oral cavity into pharynx, and the term chyme for content released from stomach into small intestine. Practically digestion commences while food is within the oral cavity, where the food is mechanically processed and mixed with saliva. Stomach The stomach is enlarged portion of digestive tube with maximal capacity to store approximately 1.5 – 2 liters of ingested food. Within its walls three layers of muscles are observed. The stomach is composed of four major parts: 1. Cardia 2. Fundus 3. Body 4. Pylorus a. Pyloric antrum b. Pyloric canal c. Pyloric sphincter Due to its curved shape, we recognize the greater and the lesser curvatures of the stomach. Mucosa of the stomach is thrown into numerous fold that are termed rugae. The glands of the stomach mucosa: 1. Surface mucous cells – secrete mucus 2. Mucous neck cells – secrete mucus – located deeper in the gastric pits 3. Parietal cells – secrete HCl acid and intrinsic factor (for vit. B12) 4. Chief cells – secrete pepsinogen and gastric lipase 5. G cells – secrete hormone gastrin